Amino acid has L- and D-enantiomers. L-amino acid is known to play an important role in forming a protein and to exhibit various bioactivities in a living body. On the other hand, although D-amino acid has been known to be constituents of a cell wall and be essential for bacteria to grow, the role of D-amino acid in eukaryotes has not been clarified. Recently, it has been known that D-amino acid has an important physiological function also in eukaryotes. Among D-amino acids, as to D-serine, there have been some reports: D-serine exists in the mammalian brain in a liberation state and acts as a coagonist of an NMDA receptor (non-patent document 1); in the cerebral spinal fluid or the serum of patients with schizophrenic disorder, the D-serine content is significantly reduced and the ratio of the D-serine content in the total serine content (D-serine content/total serine content) is also significantly reduced (non-patent documents 2 and 3); and the ratio of the D-serine content in the total serine content (D-serine content/total serine content) is also significantly reduced in the serum of patients with Alzheimer's disease (non-patent document 4), etc. In particular, much attention has been paid on the relationship with respect to nervous diseases. Although the application of D-serine in the future is being expected in this way, no simple quantification method exists in the current state. A quantification method of D-serine by using an enzyme has not been established. The quantification of serine has exclusively been carried out by a quantification method by HPLC or capillary electrophoresis after labeling with a fluorescence reagent, and the like, or a quantification method of serine by using gas chromatography (GC) after derivatization (non-patent document 5).    [Non-patent document 1] Lesson, P. D., and Iverson, L. L. (1994) J. Med. Chem. 37, 4053-4067.    [Non-patent document 2] Hashimoto et al. (2003) Arch. Gen. Psychiatry 60, 572    [Non-patent document 3] Hashimoto et al. (2005) Prog Neuropsychopharmacol Biol Psychiatry 29, 767    [Non-patent document 4] Hashimoto et al. (2004) Prog Neuropsychopharmacol Biol Psychiatry. 28, 385-8.    [Non-patent document 5] Sensitive Determination of D-Amino Acids in Mammals and the Effect of D-Amino-Acid Oxidase Activity on Their Amounts. Biological & Pharmaceutical Bulletin. (2005), 28, 1578    [Non-patent document 6] Characterization of the catalytic pathway for D-serine dehydratase. Evidence for variation of the rate-determining step with substrate structure. J Biol Chem. (1983) 258(9): 5379-5385.    [Non-patent document 7] D-serine dehydratase acting also on L-serine from Klebsiella pneumoniae. J Biochem. 1978 November; 84 (5): 1133-1138.